1. Field of the Invention
The present invention relates to an abrasive article comprising abrasive grains, e.g. corundum, an organic bonding agent, e.g. phenolic resin, or an inorganic, coldsetting bonding agent, e.g. phosphate bond, and an active filler.
2. Description of the Prior Art
It is already known that fillers are used in abrasive articles. In the abrasive industry, the term filler applies to the three following groups:
1. Fillers in the classic, usual sense, used for filling plastics materials.
These fillers have the following effects:
(a) Decreased necessity for resin; consequently lower costs of the resin system and, hence, of the abrasive article.
(b) Reinforcing effects and, consequently, increased stability of the bond between the abrasive grains. This effects an increase in the "bursting speed", abrasive hardness, lateral stability etc. of the abrasive article.
(c) Decrease in the bond stability, in such a way obtaining a smoother abrasion. Blunt abrasive grains break out more easily so that the self-sharpening properties of the abrasive articles are improved, however, the wear of the abrasive disks is increased.
With some fillers two effects, (a) and (b), or (a) and (c), occur at the same time.
Examples of such fillers which have been used are: wood powder, coconut shell flour, stone dust, feldspar, kaolin, quartz, short glass fibers, asbestos fibers, ballotini, surface-treated fine grain (silicon carbide, corundum etc.), pumice stone, cork powder etc.
It is a common feature of these fillers that they are "inactive", i.e. they do not undergo any chemical reaction or physical change during the abrasive process and therefore do not create any positive effect on the grinding process.
2. Fillers influencing the making of the grinding disks, particularly the thermal curing of the synthetic resins, are e.g. magnesium oxide, and calcium oxide.
3. "Active fillers". They undergo chemical reaction or physical change during the abrasive operation, which have a positive influence on the behaviour of the abrasive. These fillers should particularly cause an increase in the service-life of the abrasive tool and a decrease in the heating of the workpiece and the abrasive article and, hence, avoid thermal destruction. These fillers are the prerequiste for an economic processing, when materials that are hard to chip, such as unalloyed low carbon steels or titanium, are to be worked.
Active fillers can also obviously produce the same effects as the fillers indicated under (1) and (2) (increase or decrease in stability, influence on the curing process, etc).
In addition to the fillers, additives improving the adhesiveness between the abrasive grain and the bonding agent (e.g. coatings with silanes, or e.g. frits with fused metal oxides, ceramic coatings, etc.) may also be employed.
Other additives facilitate processing, for example, by either improving the noncaking free flowing qualities of the abrasive mix or reducing the internal friction in the pressing process. It is not necessary that these additives are active in the actual abrasive process.
The active fillers are the most important fillers in mixes for abrasive disks. Their effects can generally be divided into the three following main groups:
1. Decrease in the friction between abrasive grain and the workpiece, and between the abrasive grain and chips, i.e. the fillers and their by-products, must have the effect of high temperature lubricants or high pressure lubricants. They can thereby form a film of melted mass (e.g. cryolite) or a solid lubricating film (graphite, molybdenum sulfide).
2. Protective effect by forming a surface film on the abrasive grain, workpiece and chips. Grain destruction due to diffusion processes (e.g. spinel formation when grinding iron material containing corundum), the welding of the grit to the grain or to the workpiece, and the formation of built-up edges (covering of the grain with grit) are avoided.
3. Cooling effect in a location between the chips and abrasive grain, due to high melting or vaporization heat in an advantageous temperature range. That is, an endothermic reaction of the filler material takes place.
Particularly active fillers are, for example, halogenides (e.g. lead chloride, fluorspar, cryolite etc.), chalogenides (e.g. pyrite, antimony sulfide, zinc sulfide, molybdenum sulfide, selenides, tellurides etc.), low melting metals (e.g. lead, tin, low melting composition metals,) high pressure lubricants (e.g. graphite). In practice, lead chloride and antimony trisulfide have proved to be the best fillers in respect of service life and low temperature ("cool" abrasion).
It has been found that a filler is the more active the lower its phase change temperature (melting-, boiling-, sublimation-, decomposition point) is. Due to the processing conditions in the manufacture of abrasive articles, these temperatures cannot fall below a certain value. Moreover, chemically highly active elements or compounds, e.g. chlorine, hydrogen chloride, sulfur, sulfur dioxide etc., should be set free in the grinding process during decomposition.
Numerous substances can, however, not or only under certain circumstances be employed in practice as they are expensive (noble metal halogenides, molybdenum sulfide), toxic (arsenic-, selenium-, lead compounds) or hygroscopic and of high water solubility (numerous chlorides). They further strongly react with the uncured phenolic resin system (hygroscopic chlorides) or reduce the disk stability (e.g. graphite sulfur).
Some of these materials, e.g. metal chlorides, such as ferric chloride (FeCl.sub.3), zinc chloride, tin chloride, and potassium chloride, as well as elemental sulfur, are highly active and can favourably be employed in view of their low toxicity (high TLV) and costs. (TLV=treshold limit values).
Graphite is a well known high temperature- and high pressure lubricant, but it creates a number of adverse effects in abrasive articles. Pulverized graphite alone or in connection with usual, active fillers effects only a slight improvement of the abrasive properties. It would be advantageous to employ graphite in the form of coarse grains (grain size about the same as the size of the abrasive grain), which would be an advantage in the abrasive process. Adverse effects are created, however, by the high tendency of graphite to convert into dust or powder, when preparing the raw mixtures for the abrasive disks, and by the poor adhesion of the synthetic resins, particularly phenolic resins, to the smooth surfaces of the graphite grains, thus causing a great decrease in the stability of the abrasive article.